In a liquid cooled engine, a jacket is defined around each of its cylinders for circulating cooling liquid therein. In a normal internal combustion engine using a cylinder block and a cylinder head which are separately cast by metallic dies, it is customary to use inner cores which may be pulled out of the cylinder block from its interface with the cylinder head along a cylinder longitudinal line. In such an engine, the part of the cylinder defining a combustion chamber in cooperation with the cylinder head is known to reach a highest temperature level, and is therefore subjected to most severe thermal stresses.
In the case of a cylinder block having an annular opening around each of its cylinders at its interface with the cylinder head, it is necessary to increase the wall thickness of such a region in order to achieve a necessary rigidity of the part of the cylinder block defining the combustion chamber when the piston is near its top dead center. At the same time, not only the interface of the cylinder block with its cylinder head is required to be finished with a high precision but also the cylinder head and the cylinder block must be assembled together with a high precision to prevent leakage of cooling liquid into the combustion chamber. All these factors contributed to the increase in the cost of internal combustion engines.
Also, most liquid cooled engines are provided with radiators to remove heat from the cooling liquid, and the need for such a radiator prevented the use of a liquid cooling system in small engines due to economical considerations and space restrictions. Japanese patent laid open publication No. 1-151709 discloses a flywheel in which a heat exchange passage is defined for removing heat from the cooling liquid. Japanese utility model laid open publication No. 58-37920 discloses an internal combustion engine in which its lubricating oil is circulated in its jacket to achieve a favorable cooling effect and increase the capacity of its oil sump without increasing the size of its oil pan.
However, these prior art engines are intended for normal engines having a horizontal crankshaft, and cannot be directly applied to vertical crankshaft engines. Furthermore, they are not suitable for application to small engines due to their complex oil passage structure and jacket structure, respectively.